It is necessary to provide tension adjusting means for endless belt conveyors as the tension on such endless belts will vary depending upon the elongation of the belts and the loading of the belts. If such tension adjusting means were not provided the tension on the belts would, in many situations, tend to decrease to a point where there would be insufficient frictional engagement between the drive pulley and the belt to prevent slippage or creeping of the belt which would in turn cause undesirable jerks in the belt movement and would also result in abnormal wear of the belt when the tension is not sufficient.
In the past weighted or gravity take-ups have been used on endless belt conveyor systems in open mines to maintain the tension on the slack side or return run of the endless belt. While these weighted take-ups perform in a generally satisfactory manner there are certain disadvantages to this form of take-up. Thus, after the belt has elongated to such a point that the weighted take-up can no longer adequately take-up the slack, it is necessary to raise the weight so that a section of the belt may be taken out, the remaining belt being respliced so that the conveyor belt will now operate properly again. As the weight may be quite heavy, as for example a ton, it is generally difficult to raise the weight. In addition, safety standards require that the weight be suspended in such a manner that workmen cannot pass beneath the weight in the event that the weight should suddenly descend, which might happen if the belt were to break. In many situations it is desirable to use the space beneath the weight, and therefore the utilization of a weighted take-up may be undesirable in the design of such equipment. Furthermore, it is necessary to support the weight of such a take-up, and therefore the employment of a gravity take-up requires the use of support frame members of a greater load carrying capacity adjacent the take-up.
U.S. Pat. No. 2,725,976 issued Dec. 6, 1955 to Madeira discloses a tensioning device for belt conveyors which does not utilize the conventionally employed weighted or gravity take-up. In Madeira an idler roller is mounted on a carriage which runs parallel to the runs of the belt, the idler roller being moved by a tensioning mechanism which includes a winch controlled by a limit switch which is responsive to the tension on the return run of the belt. While this design eliminates the foregoing problems of the weighted take-up, it has inherent disadvantages. Thus the tensioning mechanism will hunt during the initial start-up of the conveyor as the tension on the endless run will vary considerably during the initial start-up. In this regard, it should be noted that at start-up the loading on the belt will vary as the torque varies. Similarly, during start-up the belt initially stretches, and until the full length of the belt is stretched to normal operating conditions it is not possible to get a proper measure of the working tension at the idler location. Furthermore, after start-up, the tensioning mechanism of Madeira will cause the tension idler to be moved as soon as the tension at the idler falls below a predetermined point which may not be desirable as there are sometimes certain momentary decreases on the tension on the belt after start-up due to belt lash which is caused when heavy loads are either suddenly imposed on the conveyor or discharged from the conveyor. Similarly, it is possible for machine vibrations to cause the limit switch to give false signals.
Other designs have been developed such as U.S. Pat. No. 2,846,054 issued Aug. 5, 1958 to Bergmann and U.S. Pat. No. 2,853,933 issued Nov. 4, 1958 to Hardy. The Hardy patent discloses the utilization of a winch mechanism acting through a carriage mounted idler which is utilized to maintain the tension on a return run of an endless belt. When the tension either exceeds or falls below a predetermined value the winch is caused to be operated by a limit switch after a suitable time delay by utilization of a time delay relay 183. While this design overcomes some of the disadvantages of the design shown in the previously mentioned Madeira patent, the tensioning mechanism of this design will still hunt during the initial start-up of the conveyor system. In addition, the design shown in Hardy will permit the carriage which supports the idler roller to move too far in one direction or the other. Bergmann, which is an improvement patent over Hardy, discloses the utilization of a limit switch which will prevent the movement of the carriage too far in one direction. However, Bergmann does not disclose any means to limit the movement of the carriage which supports the roller in the other direction.
In addition, it is possible in the foregoing patented constructions to decrease the tension on the endless belt during operation of the conveyor. It has been found in practice that it is seldom necessary to decrease the tension during operation of the conveyor. By permitting the tension to be decreased during operation of the conveyor, if the operator of the conveyor inadvertantly causes the tension to be decreased, the tension may in fact be decreased to such a point that belt slippage and undue belt wear may occur. Where the load on the conveyor belt is being moved uphill, if the tension is reduced to such an extent that frictional contact between the drive pulley and the belt is lost, the conveyor belt may even run in the reverse direction permitting the load to be discharged at the wrong end of the belt.
U.S. Pat. 3,217,863 issued Nov. 16, 1965 to Duncan discloses an endless belt conveyor tensioning mechanism including a centrifugal switch 39 which is employed to prevent the tensioning mechanism from being operative when the belt speed falls below a predetermined value, which may be about 75% of the full speed of the conveyor. Such a device is only satisfactory when the conveyor belt is designed to operate at a single fixed speed. However, where the conveyor belt is designed to operate at varying speeds through the employment of a variable speed drive, such a device is not practical.